The human tailbone, a vestigial remnant of a tail, is called the coccyx. It forms the final segment of the vertebral column and sits below the sacrum at the base of the spine.
The coccyx usually consists of three to five small, rudimentary vertebrae. Unlike the fully developed vertebrae higher in the spine, these segments may be partly or completely fused, and they lack several features such as prominent spinous processes. Their reduced form reflects the evolutionary loss of an external tail in the lineage leading to humans and other tailless apes.
Vestigial does not mean useless. The coccyx provides attachment points for muscles, tendons, and ligaments involved in the pelvic floor and movement. It can also flex slightly at its joint with the sacrum. The common mix-up is confusing it with the sacrum: the sacrum is the larger, wedge-shaped bone above it, while the coccyx is the small tailbone below.